1. Field of the Invention
The present invention relates to an organic light-emitting diode display, and more particularly, to an organic electroluminescent display which offers an improved aperture ratio by configuring a circuit pattern between neighboring subpixels in a symmetrical fashion to allow the subpixels to share signal lines.
2. Discussion of the Related Art
Flat panel displays, proposed to replace the existing cathode-ray tube displays, include liquid crystal displays, field emission displays, plasma display panels, and organic light-emitting diode displays (OLED displays).
Among them, the OLED display is a self-emissive display in which an organic light-emitting diode provided on a display panel has high luminance and low operating voltage characteristics and emits light by itself. Hence, the OLED display has a high contrast ratio and can be made super-thin. Also, the OLED display can easily implement moving images due to its short response time of several microseconds (μs), has an unlimited viewing angle, and is stable at low temperatures.
FIG. 1 is a view showing an equivalent circuit diagram of one pixel of an organic light-emitting diode display according to the related art.
As illustrated therein, one pixel of the organic light-emitting diode display may consist of two thin film transistors SWT and DRT, a capacitor C1, and an organic light-emitting diode EL.
The switching thin film transistor SWT applies a data voltage Vdata to a first node 1 in response to a scan signal Vscan, and the driving thin film transistor DRT receives a driving voltage ELVDD from a source electrode and, upon application of the data voltage Vdata to the first node N1, applies a current corresponding to a gate-source voltage Vgs to the organic light-emitting diode EL. The capacitor C1 serves to maintain the voltage applied to a gate electrode during 1 frame.
The organic light-emitting diode EL is composed of an organic emitting layer situated between a cathode and an anode, the cathode being connected to a drain electrode of the driving thin film transistor DRT, and the cathode being connected to ground ELVSS. The organic emitting layer may consist of a hole transport layer, an emissive layer, and an electron transport layer.
The organic light-emitting diode display represents the gradient of an image by adjusting the amount of current flowing through the organic light-emitting diode by means of the driving thin film transistor DRT. Picture quality is determined by the characteristics of the driving thin film transistor DRT.
However, the threshold voltage and electron mobility of the driving thin film transistor may vary with each pixel, even within the same display panel, and different amounts of current may flow through each organic light-emitting diode EL, which makes it difficult to get a desired gradient by compensation.
To solve this problem, as shown in FIG. 2, a structure with one or more sampling thin film transistors SPT added to it to apply a reference voltage Vref has been recently proposed. In this structure, a reference voltage SPT is applied to the sampling thin film transistor SPT, the threshold voltage Vth and electron mobility μ of the driving thin film transistor DRT are sensed by a second scan signal Vscan2 having a similar waveform to that of the first scan signal Vscan1, and variations in the sensed threshold voltage Vth and electron mobility μ components of the driving thin film transistor DRT are compensated for by external compensation or internal compensation.
FIG. 3 is a view showing a pixel structure of an organic light-emitting diode display with a sampling thin film transistor according to the related art.
Referring to FIG. 3, the related art organic light-emitting diode display has a plurality of pixels PX1 and PX2 arranged regularly. One pixel PX1 is divided into a plurality of subpixels, and the subpixels include an opening area with an organic light-emitting diode that emit light of red (R), green (G), or blue (B), and a circuit area 13 connected to the organic light-emitting diode, and where a plurality of thin film transistors including a sampling thin film transistor are formed. Subpixels of another pixel PX2, vertically adjacent to the pixel PX1, also include an opening area and a circuit area, and are arranged side by side in the same structure as pixel PX1.
As stated above, each circuit area includes a sampling thin film transistor for sensing the threshold voltage of a driving thin film transistor. A reference voltage Vref supplied to the sampling thin film transistor is applied via a reference voltage line 12 assigned to each pixel. The reference voltage line 12 is formed on the same layer as a data line and a power voltage line, in parallel with them, taking the aperture ratio of the pixels PX1 and PX2 into consideration.
In FIG. 3, one reference voltage line 12 is formed for three subpixels of red (R), green (G), and blue (B) by way of example. The vertically adjacent pixels PX1 and PX2 receive the reference voltage Vref via the same reference voltage line 12.
With this structure, the reference voltage line 12 cannot be connected in a way that passes through the circuit section of each subpixel as it is formed on the same metal layer as a data line, etc. As such, a contact area is formed between the vertically adjacent two pixels PX1 and PX2, and a reference connecting pattern 15 or 25 is formed on the same metal layer as a gate electrode and a gate line 17 or 27 to supply the reference voltage Vref to one electrode of the sampling thin film transistor of each subpixel.
According to this structure, the related art organic light emitting diode display requires a contact area to form a reference voltage line 12 between vertically adjacent pixels PX1 and PX2. The contact area occupies part of the opening area for each pixel and the circuit area for each pixel is formed in a limited area, thereby causing a decrease in aperture ratio.